This invention relates to an optical fiber having a core formed of either a glass or an organic polymer and a cladding layer formed of a vinylidene fluoride base ternary copolymer.
The recent developments of optical communication systems are quite remarkable, and practical applications of optical communication are extending over wide areas for various purposes including long distance and high capacity communication systems, local area networks, office automation and factory automation. Naturally, much efforts have been devoted to developments of various types of optical fibers.
Optical fibers using glasses in both the core and cladding are prevailing for long distance communication purposes. Organic plastics optical fibers, in which the core material is usually a methacrylic resin, have been developed mainly for short distance transmission purposes in view of their relative inferiority in respect of transmission loss and advantages in the ease of fabrication and processing and in the feasibility of considerabe enlargement of fiber diameters. Furthermore, recently increasing interest has been attached to composite type optical fibers having a glass core and an organic polymer cladding layer for use in short and medium distance transmissions.
For practicable optical fibers having an organic polymer cladding layer, the following items are required of the cladding material: (1) to be obtained at low cost, (2) to be sufficiently high in softening temperature, (3) to be excellent in processability as fiber, (4) to be good in adhesion to the core material, (5) to be sufficiently high in flexibility, (6) to be excellent in weatherability, (7) to be low in moisture absorption, (8) to be high in transparency and (9) to be low in refractivity. There are not many kinds of organic polymers that fully satisfy the above listed requirements.
Besides silicones, some fluororesins have been tested as cladding materials for optical fibers since fluororesins are inherently low in refractivity and excellent in weatherabiltiy. For example, JP-A No. 51-52849 shows using a copolymer of vinylidene fluoride (abbreviated to VDF) with tetrafluoroethylene (TFE) as the cladding material for an optical fiber having a glass core, and JP-A No. 53-60242 shows using a copolymer of VDF with trifluoroethylene (TrFE) as the cladding material for an optical fiber having a polymethyl methacrylate (PMMA) core. Such VDF base copolymers are obtained at relatively low costs. However, as the cladding material the VDF/TFE and VDF/TrFE copolymers are not fully satisfactory particularly in respect of transparency and flexibility.
GB-A No. 2,161,954 and GB-A No. 2,171,219 show optical fibers using a binary copolymer of VDF with hexafluoroacetone (HFA), which is better than VDF/TFE and VDF/TrFE copolymers in both transparency and flexibility. However, even these optical fibers are not deemed ideal in every respect.